How big is Hourglass Nebula? | Scale of the Universe

The Hourglass Nebula, known to astronomers as MyCn 18, is a cloud of glowing gas shaped like an hourglass, thrown off by a dying star in the constellation Musca. From end to end it is about 6.6 to 8.3 trillion kilometers long. That is 0.7 to 0.9 light-years, where a light-year is the distance light travels in a year. The range comes from uncertainty in the nebula's distance from Earth.

Size

MyCn 18 is a planetary nebula: the outer layers of a Sun-like star, blown off near the end of its life and glowing around the small, hot core left behind. Its gas forms two lobes joined at a narrow waist. A 2014 study built a three-dimensional model of this shape from telescope images and from measurements of how the gas moves. The model is 18 arcseconds from end to end, where an arcsecond is 1/3,600 of a degree.

In the same model, each lobe is up to 8.5 arcseconds wide, and the waist is only 1.8 arcseconds across. Turning these angles into lengths needs the nebula's distance, which is hard to measure. The figure usually quoted, about 8,000 light-years, comes from a statistical method. Later work found that this method gives distances that are too small for nebulae of this shape.

A 2018 study combined two newer estimates and found about 10,000 light-years, give or take about 16%. At 8,000 light-years the hourglass would be about 6.6 trillion kilometers long, and at 10,000 light-years about 8.3 trillion.

Size comparisons

The hourglass is between about 44,000 and 56,000 times the average distance from Earth to the Sun.
It is roughly three-quarters of a light-year long.
It is about four times the distance from Proxima Centauri to Alpha Centauri A.
Its narrow waist is about 4,400 to 5,600 times the Earth–Sun distance across, about 70 to 90 times the width of Neptune's orbit.
If the hourglass were shrunk to the height of a person 1.7 meters (5 feet 7 inches) tall, Earth's whole orbit around the Sun would be only 0.06 to 0.08 millimeters across, less than the width of a human hair.

The etched hourglass

In pictures taken from the ground, MyCn 18 looked like a pair of large rings with a smaller ring between them. In 1996, the Hubble Space Telescope showed its true shape: an hourglass with an intricate pattern of "etchings" in its walls. The team that studied the images named it the Etched Hourglass Nebula. In Hubble's 1996 picture, red shows glowing nitrogen, green hydrogen and blue oxygen.

How it formed

One explanation for the shape is that a fast wind from the dying star expanded inside a slower cloud of gas that was denser around its middle than near its poles. By one estimate, the hourglass was thrown off about 2,700 years ago.

In 2018, astronomers found that the central star has a companion, a small red dwarf star that circles it every 18 days. The discovery supports a long-suspected link between hourglass-shaped nebulae and pairs of stars that interact. The central star also sits slightly off the nebula's center. The same study suggests this may be because the star is moving through space.